The maize mutant iojap has been a model system in the study of nuclear-plastid interactions in higher plants since the work of Jenkins (1924) and of Rhoades (1943). Plants homozygous for this nuclear mutation have variegated leaves, and transmit defective chloroplasts through their female gametes. Maternally inherited defective plastids have suffered an irreversible change that cannot be rescued by the nuclear genotype of the zygote, resulting in albino seedlings whose plastids are incapable of differentiation into chloroplasts. Furthermore, the pattern of leaf variegation observed in homozygous individuals suggests that the gene product is a key component of the regulatory machinery that co-ordinates cellular and plastid development in higher plants. We have recently obtained a molecular clone of the iojap locus by transposon-tagging using the maize transposon Mu1. In this proposal we describe experiments intended to determine the role of the Iojap gene product in nuclear-plastid interaction and the basis for the maternal inheritance of the defective plastids. We shall also initiate genetic studies aimed at determining the developmental parameters involved in the patterns of leaf variegation observed in mutant plants. The proposed study will contribute to our understanding of the mechanism by which nuclear-encoded proteins regulate chloroplast biogenesis during leaf development in higher plants. It might also provide a molecular explanation for the patterns of leaf variegation and maternal inheritance observed in this and other higher plant mutants of this type. We hope to distinguish between a number of genetic mechanisms that have been postulated to account for these patterns, and so further our understanding of pattern formation in plants and other multicellular organisms.